The GPU

Tegra 4 features an evolved GPU core compared to Tegra 3. The architecture retains a fixed division between pixel and vertex shader hardware, making it the only modern mobile GPU architecture not to adopt a unified shader model.

We finally have a competitive GPU architecture from NVIDIA. It’s hardly industry leading in terms of specs, but there’s a good amount of the 80mm^2 die dedicated towards pixel and vertex shading hardware. There's also a new L2 texture cache that helps improve overall bandwidth efficiency.

The big omission here is the lack of full OpenGL ES 3.0 support. NVIDIA’s pixel shader hardware remains FP24, while the ES 3.0 spec requires full FP32 support for both pixel and vertex shaders. NVIDIA also lacks ETC and FP texture support, although some features of ES 3.0 are implemented (e.g. Multiple Render Targets).

Mobile SoC GPU Comparison

GeForce ULP (2012)

PowerVR SGX 543MP2

PowerVR SGX 543MP4

PowerVR SGX 544MP3

PowerVR SGX 554MP4

GeForce ULP (2013)

Used In

Tegra 3

A5

A5X

Exynos 5 Octa

A6X

Tegra 4

SIMD Name

core

USSE2

USSE2

USSE2

USSE2

core

# of SIMDs

3

8

16

12

32

18

MADs per SIMD

4

4

4

4

4

4

Total MADs

12

32

64

48

128

72

GFLOPS @ Shipping Frequency

12.4 GFLOPS

16.0 GFLOPS

32.0 GFLOPS

51.1 GFLOPS

71.6 GFLOPS

74.8 GFLOPS

For users today, the lack of OpenGL ES 3.0 support likely doesn’t matter - but it’ll matter more in a year or two when game developers start using OpenGL ES 3.0. NVIDIA is fully capable of building an OpenGL ES 3.0 enabled GPU, and I suspect the resistance here boils down to wanting to win performance comparisons today without making die size any larger than it needs to be. Remembering back to the earlier discussion about NVIDIA’s cost position in the market, this decision makes sense from NVIDIA’s stance although it’s not great for the industry as a whole.

Tegra 4i retains the same base GPU architecture as Tegra 4, but dramatically cuts down on hardware. NVIDIA goes from 4 down to 3 vertex units, and moves to two larger pixel shader units (increasing the ratio of compute to texture hardware in the T4i GPU). The max T4i GPU clock drops a bit down to 660MHz, but that still gives it substantially more performance than NVIDIA’s Tegra 3.

Memory Interface

The first three generations of Tegra SoCs had an embarrassingly small amount of memory bandwidth, at least compared to Apple, Samsung and Qualcomm. Admittedly, Samsung and Qualcomm were late adopters of a dual-channel memory interface, but they still got there much quicker than NVIDIA did.

With Tegra 4, complaints about memory bandwidth can finally be thrown out the window. The Tegra 4 SoC features two 32-bit LPDDR3 memory interfaces, bringing it up to par with the competition. The current max data rate supported by Tegra 4’s memory interfaces is 1866MHz, but that may go up in the future.

Tegra 4 won’t ship in a PoP (package-on-package) configuration and will have to be paired with external DRAM. This will limit Tegra 4 to larger devices, but it should still be able to fit in a phone.

Unfortunately, Tegra 4i only has a single channel LPDDR3 memory interface. Tegra 4i on the other hand will be available in PoP as well as discrete configurations. The PoP configuration may top out at LPDDR3-1600, while the discrete version can scale up to 1866MHz and beyond.

I wouldn't expect a huge downclock for phones , they do need to limit heat, not going with POP for the RAM helps ,some actual cooling (air gap or metal) could also be used so they will most likely allow 1-2 cores to go pretty high and maybe all 4 for short periods of time (so the usual tricks to get more out of it).Reply

"At the same time, the elephant in the room is OpenCL (and its current absence on Tegra 4) and what direction the industry will take that to leverage GPU compute for some computational photography processing."Reply

The Icera acquisition was a brilliant one. This gives NVidia the complete mobile package. It will be very interesting to see how this works out in practice. NVidia is a fierce competitor, Qualcomm should be worried. Reply